Westpac has blamed a software patch for website outages that left customers unable to access the bank's online account services for much of yesterday.

Customers attempting to access their accounts on Westpac's website yesterday were greeted with long delays or were unable to gain access at all. Westpac yesterday apologised to customers affected by the outage, explaining that a patch update put through by the bank's IT department had to be rolled back before services resumed as normal.

"Apparently what had occurred the evening before was that they'd put in a patch [on the server] and we believe that patch caused a problem and that patch had to be backed out," Westpac spokesperson David Lording told ZDNet Australia.

"It did take longer than these things usually would, but it was all up and going at about 5pm," he added.

Lording denied reports that the website downtime was related to a security issue and could not say whether the bank would attempt to reapply the Authorised Program Analysis Report (APAR) patch that caused all the woes for the bank's website in the near future.

The outage was not the first for an Australian bank this week, with a security-related upgrade causing the Commonwealth Bank of Australia's ATM and EFTPOS network to go down on Monday.